You know, there's an unusual thing about driving for Uber. Your pay is constantly changing. It's dynamic. The company dials the fare up and down according to supply and demand. Economists, of course, marvel at this superpower. Setting price in real time is something you couldn't do before the smartphone.

Drivers, though, scratch their heads because they're just not sure what they're making. As NPR's Aarti Shahani discovered, it can lead some drivers to stay on the road many more hours than is actually safe.

AARTI SHAHANI, BYLINE: NPR surveyed hundreds of Uber drivers. Seventy-nine of them say that, to hit an earnings goal or finish a far-out-of-the-way ride, they've driven shifts 14 hours or longer. That means being in the car, Uber app on, either driving a passenger or waiting for work.

Three people showed NPR documentation for driving about 20 hours in one day. Uber places no limit on how long you can drive. This is clearly a public safety hazard. But Uber drivers talk about it like it's normal.

KYLE RENINGER: Well, I'm getting started a little late here.

SHAHANI: It's Friday, 5:06 p.m., time for Kyle Reninger to cash in as an Uber driver. He thinks he can make $200 tonight.

RENINGER: So I chose to take a nap in the middle of the day so that I could stay up later tonight and drive.

SHAHANI: The 32-year-old from Muncie, Ind., is keeping an audio diary for NPR over several weeks so we can peer into his modern workplace. He holds his phone or uses speaker phone depending on if he's driving or not, so you'll hear the difference.

Reninger drives weekend nights because he finds that's when you're most likely to pick up surge fares, the higher prices - two, three, five times the regular rate that passengers hate to pay and that drivers live by, the nuggets of the Uber gold rush. Reninger started a Facebook group for Muncie drivers to share advice, tricks. He says people have to enjoy this job.

SHAHANI: Reninger feels like his own boss, and Uber is his side gig. He and his wife have a family business selling vegan baked goods. So he loads his car with vegan cookies and gets to market a bit to his passengers. He's on Uber and Lyft, a competitor, though the vast majority of his business comes from Uber.

RENINGER: I got my first request for the night. It took a little over an hour to get my first request - so not too bad.

SHAHANI: That's one hour driving around with his app on. The best drivers have a lay of the land and a strategy. Tonight, Reninger notices on his Facebook group that a lot of other drivers are staying local. So he decides to head south, a little more than an hour away, to Indianapolis. He says his first passenger, Diane (ph), was a nice lady. But...

RENINGER: The bad part of it was that I checked how much the estimated trip would've cost. And Uber says 25. And my driver app says that she was charged 18. So there's a obvious discrepancy there.

SHAHANI: Minus the Uber fee, he makes $14.14, which is not great after one hour and 38 minutes on the road. Reninger says he's used to Uber playing this game, estimating one fare and paying another. He sends Uber a message, basically saying, hey, I got underpaid $7 here. Fix that. Then he keeps on waiting for his next passenger.

RENINGER: This is the hard part, the in-between. Not really sure, you know, if I should drive around and hope to get another ping or if I should head back north.

SHAHANI: And in between the waiting, the passengers he manages to get make him laugh, think, cringe - 9:30 p.m.

RENINGER: Good conversation with the last rider - interesting. He's a world traveler, just got back from Amsterdam.

SHAHANI: 11:45 p.m.

RENINGER: Then you have the one jerk for the night.

SHAHANI: Drunk guy with an open beer in his hand. Reninger says, you can't bring it in.

RENINGER: Said, sorry, man. It's the law. He's like, whatever.

SHAHANI: Drunk guy motions like he's setting the can down in the street. But it turns out he didn't.

RENINGER: So that was kind of a jerk move.

SHAHANI: 1:42 a.m.

RENINGER: About halfway through the night, I guess - or more than half way - and only about halfway to my goal, though.

SHAHANI: Again, he was hoping for $200. 2:43 a.m. - another drunk guy.

RENINGER: He asked me my name about 12 times. And he kept saying, that's a great name. That's a great name. And I'm really drunk, man. I'm really drunk.

SHAHANI: And it didn't end there. Reninger took him to IHOP.

RENINGER: I dropped him off there - got out of the car, zipped down his pants and started to urinate right in front of IHOP and then just walked in like nothing happened.

SHAHANI: 3:36 a.m. - his first surge fare for the night.

RENINGER: And it was only a 1.8-point surge. But even that makes such a huge difference.

SHAHANI: 6:11 a.m. - an airport run - actually when he coordinated with his first passenger, that nice lady Diane.

RENINGER: And she tipped me ten bucks cash, which was nice of her.

SHAHANI: Uber, by the way, rejected his claim in the $7 dispute. You can start to hear Kyle Reninger's voice fading. And there's something he's not admitting right now. He waits until he gets home to mention he was having a hard time behind the wheel.

RENINGER: Just listen to more comedy and roll the window down. Sip on water. Just try to stay awake. I made it, though. And I'm going to go crash for a while.

SHAHANI: All told, Kyle Reninger was in his car 14 hours, nine minutes. He covered a total of 401.2 miles. If you put aside the cost of gas, he made $165.30. Plus tips, he nearly hit his goal of $200. It just took a couple more hours than he'd hoped, but it was by no means his longest shift.

And so it goes. Each night is a gamble. The hours go by just like in a casino. Only here, hooked by ambition and uncertainty, you could get tired and kill someone. One final note - Uber could design its app to lock out drivers after a certain number of hours, but a company spokesperson says in an e-mail, Uber is a flexible work opportunity.

Also, the company is testing out a new app alert to remind drivers about the importance of getting enough rest. Aarti Shahani, NPR News, San Francisco.